It is well known to deposit aqueous inks, particularly those having anionically stabilized pigments on a substrate having multivalent metal salt on the surface of the substrate in order to prevent the ink drops from either penetrating too far below the surface of a substrate such as an uncoated paper, thereby resulting in a lowering of optical density, or from bleeding or coalescing of adjacent ink drops of the same or different colors on a less absorbent substrate, thereby resulting in blurry or grainy appearing images. Surface treatments comprising multivalent metal ions are particularly advantageous for high speed printing with page-wide ink jet arrays, wherein adjacent drops of ink are deposited within just a few microseconds of each other onto the substrate.
Dannhauser, et al. (US 2011/0279554), describe an inkjet receiving medium suitable for high speed inkjet printing which includes a substrate having a topmost layer coated thereon comprising an aqueous soluble salt of a multivalent metal cation and a cross-linked hydrophilic polymer binder.
Xiang and Botros (U.S. Pat. No. 8,562,126 B1) describe an inkjet receiving medium comprising a substrate and having a topmost layer coated thereon, wherein the topmost layer includes one or more aqueous soluble salts of multivalent metal cations, a cationic polyelectrolyte comprising amidine moieties, and a second polymer which is distinct from the cationic polyelectrolyte comprising amidine moieties and which is selected from the group including a polyamide-epichlorohydrin, a polyamine solution polymer, and a waterborne or water-dispersible polyurethane. Although images printed on substrates with these types of surface treatments display excellent image quality and adequate durability for many applications, it has been found that immediately after printing and for some time thereafter, the printed inks can still be susceptible to smudging or offsetting.
In an attempt to address these issues, Dannhauser, et al., (WO2013/165882), disclose adding particles comprised primarily of a polymer having a Rockwell Hardness of less than R90 and having a mode equivalent spherical diameter of at least about 2 micrometers to coating compositions comprising one or more aqueous soluble salts of multivalent metal cations.
Other approaches to improving the durability of printed substrates involve the application of a protective post-coating, such as a varnish or laminate. For example, Sarkisian, et al., (US 2011/0303113 A1) disclose the application of overprint varnishes to substrates having multivalent metal salts that have been printed with anionically stabilized pigment inks before application of the varnish. These varnishes are claimed to protect the inks from wet or dry abrasion. However, there is no mention of the impact of the varnish on the durability of the ink layer itself, e.g., ink cohesion or ink adhesion to the substrate. Another obvious drawback of this approach is that it can add substantial cost and complexity to the process and the printed article.
Although these coatings are improved in certain aspects of durability, the need remains for more durable digital prints to overcome the effects of handling in production and in use by the consumer. In particular, there is a need to improve the physical durability of the printed ink on the substrate as soon as possible after drying and exiting the printing press. There is also the need to improve the adhesion of laminates or other types of overcoats applied shortly after printing.
It has been observed that aqueous inks comprising anionically stabilized pigments deposited on a substrate having a multivalent salt on the surface exhibit relatively poor durability immediately after drying, but they generally increase in durability with aging over an extended period of time. Although ambient aging does result in increased durability, the results are inconsistent and unpredictable due to variations in the ambient conditions. Furthermore, to attain adequate levels of durability for many applications, the time required under ambient conditions is unacceptably long. Thus there is a need to accelerate the aging process such that much improved print durability is achieved as soon as possible after printing and drying.